Thermal flow sensor having recesses in a substrate

ABSTRACT

A thermal flow sensor has a first, second and third substrate, each having a first side and a second opposite side. The first substrate is connected to the second substrate such that the second side of the first substrate abuts the first side of the second substrate. The third substrate is connected to the second substrate such that the second side of the second substrate abuts the first side of the third substrate. The second substrate has a groove formed therein so as to form a conduit bounded by the second substrate and the second side of the first substrate and the first side of the third substrate. A heater is disposed on the first side of the first substrate opposed to the conduit. A first temperature sensor is disposed on the first side of the first substrate opposed to the conduit. A second temperature sensor is disposed on the first side of the first substrate opposed to the conduit. A first recess is formed in at least one of the first side and the second side of the first substrate between the heater and one of the first temperature sensor and the second temperature sensor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a thermal flow sensor. Moreparticularly, the present invention relates to a thermal flow sensorthat can be used to monitor the flow of cerebrospinal fluid (CSF) withina shunt.

2. Discussion of Related Art

Hydrocephalus is a condition caused by an abnormal accumulation of CSFin cavities inside the brain. If not treated properly, hydrocephalus cancause severe disablements in children and adults, and can even causedeath. If cerebrospinal fluid accumulates, the ventricles becomeenlarged and the pressure inside the brain increases. Hydrocephalus is asevere degenerative condition that occurs in children from birth on.Hydrocephalus is presumably caused by a complex interaction betweengenetic and environmental factors. A person can also acquirehydrocephalus later in life, which can be due to, for example, spinabifida, brain hemorrhage, meningitis, skull trauma, tumors and cysts.

Hydrocephalus occurs in newborns with a frequency of approximately 1 outof 5,000-10,000. There is currently no known prevention or cure forhydrocephalus. The most effective treatment so far is the surgicalimplantation of a shunt behind the ear. A shunt is a flexible tube thatis inserted into the ventricular system of the brain to divert thecerebro fluid to other regions of the body. However, shunts frequentlymalfunction, leading to infections which can cause severe complicationsfor the patient (e.g., delayed development, learning disabilities).

According to some estimates, up to 50% of patients who receive a shunt,will have the shunt malfunction at some time during his or her lifetime.Most shunt malfunctions are due to a blocked catheter and an incorrectlyadjusted shunt valve.

The present inventors believe that the occurrence of complications dueto a shunt malfunction can be detected easier by using a miniaturizedimplantable flow sensor, in accordance with the present invention, thathas been developed for monitoring CSF flow. The sensor employstemperature sensors and a heater that do not contact the CSF, yetmeasures the CSF flow and can therefore be implanted so as to last foran extended period of time (e.g., greater than 10 years).

In particular, when a shunt valve is implanted in children, amalfunction of the implant can be effectively detected by use of anadditional implanted sensor.

The thermal flow sensor in accordance with the present inventionrepresents a significant advance in the treatment of hydrocephalus inpatients and also represents an additional step towards the developmentof a closed-loop control system, which can continuously optimize theflow rate in the patient's shunt valve.

In addition, the thermal flow sensor of the present invention providesphysicians with novel, previously unattainable information about theformation and drainage of cerebro spinal fluid (CSF).

SUMMARY OF THE INVENTION

In accordance with a currently preferred exemplary embodiment, thepresent invention involves athermal flow sensor having a first substratehaving a first side and a second opposite side. A second substrate has afirst side and a second opposite side. The first substrate is connectedto the second substrate such that the second side of the first substrateabuts the first side of the second substrate. A third substrate has afirst side and a second opposite side. The third substrate is connectedto the second substrate such that the second side of the secondsubstrate abuts the first side of the third substrate. The secondsubstrate has a groove formed therein so as to form a conduit bounded bythe second substrate and the second side of the first substrate and thefirst side of the third substrate. A heater is disposed on the firstside of the first substrate opposed to the conduit. A first temperaturesensor is disposed on the first side of the first substrate opposed tothe conduit. A second temperature sensor is disposed on the first sideof the first substrate opposed to the conduit. A first recess is formedin at least one of the first side and the second side of the firstsubstrate between the heater and one of the first temperature sensor andthe second temperature sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of a specific embodiment thereof,especially when taken in conjunction with the accompanying drawingswherein like reference numerals in the various figures are utilized todesignate like components, and wherein:

FIG. 1 is a perspective view of the thermal flow sensor in accordancewith the present invention;

FIG. 2 is a cross-sectional schematic view taken along line 2-2 of FIG.1 and looking in the direction of the arrows;

FIG. 3 is a cross-sectional view taken along line 3-3 of FIG. 2 andlooking in the direction of the arrows;

FIG. 4A is a cross-sectional view similar to FIG. 2 showing the thermalflow sensor having only two substrates with the groove formed in thesecond substrate;

FIG. 4B is a cross-sectional view similar to FIG. 2 showing the thermalflow sensor having only two substrates with the groove formed in thefirst substrate;

FIG. 4C is a cross-sectional view similar to FIG. 2 showing the thermalflow sensor having only two substrates with the groove formed in boththe first and second substrate;

FIG. 5 is a cross-sectional view similar to FIG. 2 showing the thermalflow sensor having only one substrate with the groove formed therein;

FIG. 6 is a cross-sectional view taken along line 6-6 of FIG. 5 andlooking in the direction of the arrows;

FIG. 7 is an enlarged partial perspective view of the first substrateand the heater and two temperature sensors mounted on the upper surfaceof the first substrate;

FIG. 8A is a partial cross-sectional view of the thermal flow sensorshowing the recesses in the first side of the first substrate;

FIG. 8B is a partial cross-sectional view of the thermal flow sensorshowing the recess in the second side of the first substrate;

FIG. 8C is a partial cross-sectional view of the thermal flow sensorshowing one of the recesses on the first side of the first substrate andthe other recess on the second side of the first substrate;

FIG. 8D is a partial cross-sectional view of the thermal flow sensorshowing the recesses on the first side of the first substrate and on thesecond side of the first substrate;

FIG. 9A is a cross-sectional view of the thermal flow sensor showing anasymmetric design of the temperature sensors upstream from the heater;

FIG. 9B is a cross-sectional view of the thermal flow sensor showing anasymmetric design of the temperature sensors downstream from the heater;

FIG. 10A is a cross-sectional view of the thermal flow sensor showing anasymmetric design of the temperature sensors upstream from the heaterand within the conduit;

FIG. 10B is a cross-sectional view of the thermal flow sensor showing anasymmetric design of the temperature sensors downstream from the heaterand within the conduit;

FIG. 11 is a perspective view showing the thermal flow sensor beingincorporated within a shunt; and

FIG. 12 is a schematic plan view of the first or second side of thefirst substrate showing the heater and temperature sensors.

DETAILED DESCRIPTION OF THE CURRENTLY PREFERRED EXEMPLARY EMBODIMENT

Referring now to FIGS. 1 though 6, a thermal flow sensor 10 inaccordance with the present invention is illustrated. The thermal flowsensor in a currently preferred exemplary embodiment includes a firstsubstrate 12, a second substrate 14 and a third substrate 16. Firstsubstrate 12 has a first side 18 and a second opposite side 20. Secondsubstrate 14 has a first side 22 and a second opposite side 24. Thirdsubstrate 16 has a first side 26 and a second opposite side 28. Firstsubstrate 12 is connected to second substrate 14 such that the secondside 20 of the first substrate 12 abuts the first side 22 of the secondsubstrate 14. Third substrate 16 is connected to the second substrate 14such that the second side 24 of the second substrate 14 abuts the firstside 26 of the third substrate. The first substrate is preferably bondedto the second substrate, and the second substrate is preferably bondedto the third substrate. The first and third substrates are preferablymade of borosilicate glass, for example PYREX® or BOROFLOAT®. The secondsubstrate is preferably made of silicon.

Second substrate 14 has a groove 30 formed therein so as to form aconduit 32 bounded by the second substrate 14 and the second side 20 ofthe first substrate and the first side 26 of the third substrate, asillustrated in FIG. 1. The groove is preferably formed by etching intothe silicon second substrate 14. In one exemplary embodiment, the groovemay have a cross-sectional dimension of 380 μm×3000 μm. A heater 34 isdisposed on the first side 18 of the first substrate 12 opposed toconduit 32. A first temperature sensor 36 is disposed on the first side18 of said first substrate 12 opposed to conduit 32. A secondtemperature sensor 38 is also disposed on the first side 18 of the firstsubstrate 12 opposed to conduit 32. This sensor can detect a temperaturedifference of approximately 0.005° C. at a flow rate of 300 ml/hr.

The temperature sensors and heater are preferably created by metaldeposition (e.g., evaporation or sputtering) directly on the first sideor second side of the first substrate, which is preferably made ofborosilicate glass. These metal deposition processes permit one todeposit thin films of metal on the glass surface within a vacuumchamber. A person skilled in the art will readily understand how topattern the thin films by lithographic processes. In one exemplaryembodiment, the metal thin film is made of several layers (e.g.,Chromium (Cr), Platinum (Pt), Titanium (Ti) and Gold (Au)). Chromium orTitanium is preferably used as an adhesion layer since it sticks well tothe borosilicate glass. Afterwards, a layer of Pt is deposited on the Cror Ti so that it may be used as the heater and temperature structures.One may also at the same time as when the heater and temperature sensorsare created, create the electrical tracks for the remainder of theelectronics on the same substrate. A gold layer is preferably depositedon top of the platinum only in the region where there is no heater ortemperature sensor structure and serves as the electrical tracks for therest of the electronic circuit. However, in the region where there is noheater or temperature sensor, the gold layer could be deposited directlyon the adhesion layer of Cr or Ti. The heater works by resistive heatingby passing a current therethrough, as shown in FIG. 12. The temperaturesensors work by having their resistance change due to its ambienttemperature, as also shown in FIG. 12. In the present invention sensors,the ambient temperature at each temperature sensor is dependent uponamong other things, the amount of heat created by the heater, thethickness of the first substrate, and the flow rate of the fluid flowingthrough the conduit.

A cap 40 is mounted on the first side 18 of the first substrate 12,thereby forming an interior chamber 42. Cap 40 is preferably made ofPYREX® a and is brazed to the first substrate, thereby forming ahermetically sealed interior chamber 42. When the sensor is used as animplantable medical device, a final parylene layer is applied on theouter surface of the sensor to prevent rejection of the implant by thebody. Heater 34, first temperature sensor 36 and second temperaturesensor 38 are disposed within interior chamber 42. Other electronics 44are also disposed within chamber 42 and are electrically connected toheater 34, first temperature sensor 36 and second temperature sensor 38.One skilled in the art will readily know how to assemble the electronicsso that data from the heater and/or sensors can be communicated bytelemetry to and from an external control unit. By placing thetemperature sensors and the heater on the opposite side of the firstsubstrate from the conduit, the sensors and heater are not in directcontact with the fluid (e.g., CSF) within the conduit. This structure isreferred to as an inverted substrate. Thus, the sensor in accordancewith the present invention is a biocompatible design, which is favorablefor long-term implants such as a hydrocephalus shunt, an infusion pump(e.g. >10 years). The biocompatible packaging of the sensor and theelectronics has at least the following advantages:

-   -   The body fluid comes in contact only with biocompatible glass.    -   The Ti/Pt sensors, heater and sensor electronics are located on        the same substrate, which reduces their manufacturing cost.    -   The sensor electronics can be drastically miniaturized by        employing an ASIC, which can be fabricated by flip-chip        technology.

In accordance with an alternative embodiment, the thermal flow sensorcan comprise of two substrates 12′ and 14′, with a groove 30′ formedwithin either substrate or both to form a conduit 32′ bounded by bothsubstrates, as illustrated in FIGS. 4 a, b and c. In another alternativeembodiment, the thermal flow sensor can comprise of only one substrate12″, as shown in FIGS. 5 and 6. Substrate 12″ has a first upper side 18″and a second opposite lower side 20″, and at least one side edge 46″extending between first upper side 18″ and second lower side 20″. Aconduit 32″ is formed within substrate 12″. Conduit 12″ has an inletopening 48″ and an outlet opening 50″. Each of the openings 48″, 50″ areformed in the at least one edge 46″, as shown in FIG. 3.

To determine the flow rate of a fluid flowing within conduit 32, 32′,32″, fluid is permitted to flow through the conduit by entering into theinlet opening of the conduit and exiting from the exit opening. Thefluid is heated with the heater 34 opposed to and remote from theconduit. In other words, the heater and temperature sensors are not incontact with the fluid flowing within the conduit. The temperature ofthe fluid is detected with the first temperature sensor disposed on thefirst side of the body opposed to and remote from the conduit. Thetemperature of the fluid may also be detected with the secondtemperature sensor disposed on the first side of the body opposed to andremote from the conduit. In a currently preferred exemplary embodiment,the two temperature sensors are spaced apart by about 2000 μm. Thespacing between the temperature sensors is in part dependent upon theflow rate to be measured. Based on the detected temperature(s), the flowrate of the fluid can readily be determined by one skilled in the art.The fluid is preferably CSF, and thermal flow sensor 10 is preferablydisposed within shunt 100, as shown in FIG. 10.

In designing the sensor in accordance with the present invention, thesensor was optimized through static and dynamic FEM simulations for flowranges reaching 300 ml/hr, with optimized sensitivity at a flow range of25 ml/hr, and for rapid step responses of 2 seconds. The normal flowrange of CSF is about 25 ml/hr. At a flow range of 25 ml/hr, thesensitivity of the sensor signal is about 140 mV/ml/hr; and for highflow ranges of >270 ml/hr, the sensitivity of the sensor signal is stillabout 5 mV/ml/hr. The response time of the sensor of about 2 sec. isconsiderably reduced as compared to about 10 sec. for conventionalsensors on a glass substrate. In addition, these conventional sensorscan only detect flow rates up to 2-3 ml/hr. The fast step response makesit possible to measure CSF flow even when the patient's head positionchanges rapidly (e.g., when arising, or getting up from sleeping, etc.).

Referring now to FIG. 3, the first, second and third substrate togetherform a multi-layer body structure that has at least one edge 46extending between the first side 18 of the first substrate and saidsecond side 28 of the third substrate. Conduit 32 has an inlet opening48 and an outlet opening 50, each of which are formed in the at leastone edge 46. In a currently preferred exemplary embodiment, inletopening 48 and outlet opening 50 are disposed solely in the secondsubstrate 14. A dicing saw may be used to cut through the three layersto expose the openings in the second substrate. This embodiment isreferred to as a streamline packaging because the inlet and outletopenings are in the side edges of the body structure as opposed to thetop and/or bottom surface.

Referring now to FIG. 7, in accordance with another embodiment of thepresent invention, a first recess 52 is formed in the first side 18 ofthe first substrate 12 between heater 34 and the first temperaturesensor 36. As shown, first recess 52 is disposed immediately adjacent toheater 34. A second recess 54 is formed in the first side 18 of thefirst substrate 12 between heater 34 and the second temperature sensor38 (see FIG. 8A). As shown, second recess 54 is disposed immediatelyadjacent to heater 34 on an opposite side of the heater from the firstrecess. Alternatively, as shown in FIGS. 8B and 8C, the recesses 52, 54can be formed in the second side of the first substrate 12 or one on oneside of the first substrate and the other on the second side of thefirst substrate, respectively. Recesses 52, 54 preferably extend intothe first substrate for about half of the thickness of the firstsubstrate. In accordance with another variation of the presentinvention, the recesses 52, 54 can be disposed on the first side of thefirst substrate and on the second side of the first substrate.

The recesses 52, 54 are used to help guide the heat generated by heater34 through the first substrate, as indicated by arrows A, and intoconduit 32. The heat energy absorbed by the fluid is then transferredback through the first substrate, as indicated by arrows B, to the firstand second temperature sensors. Because air is not a very good conductorof heat, most, if not effectively all, of the heat generated by theheater travels along the path indicated by arrows A and B. Of course,some heat will travel through the first substrate, but one of skill inthe art will readily be able to calibrate the thermal flow sensor inaccordance with the present invention to take this factor into account.Depending upon the thickness of the first substrate, how much heat isgenerated by the heater, the dimension of the recesses, and otherfactors known to those skilled in the art, one can readily determine theflow rate of the fluid flowing through the conduit. This information canthen be transmitted by telemetry to an external control unit (notshown).

As in the previous alternative embodiments shown in FIGS. 4A-5, thethermal flow sensor having recesses 52, 54 can also be comprised of twosubstrates 12′ and 14′, as illustrated in FIGS. 4 a, b and c, or withonly one substrate 12″, as shown in FIG. 5.

Referring now to FIG. 9A, a thermal flow sensor in accordance with yetanother embodiment of the present invention is illustrated. In thisembodiment, first temperature sensor 36 is disposed on the first side ofthe first substrate opposed to the conduit and at a first predetermineddistance from heater 34 in a direction opposite to the fluid flowdirection within the conduit. Second temperature sensor 38 is disposedon the first side of the first substrate opposed to the conduit and at asecond predetermined distance from heater 34 in a direction opposite tothe fluid flow direction. As illustrated in FIG. 9A, the secondpredetermined distance is greater than the first predetermined distance.This embodiment is referred to as an asymmetric sensor design becauseboth temperature sensors are disposed on one side of the heater, asopposed to having the heater being disposed between the two temperaturesensors with respect to the flow direction.

Referring now to FIG. 9B, a variation of the embodiment of FIG. 9 isillustrated. In this variation, the first and second temperature sensorsare disposed at a respective first and second predetermined distancefrom the heater in the fluid flow direction, as opposed to opposite tothe fluid flow direction.

Referring now to FIGS. 10A and 10B, another variation of the embodimentof FIG. 9 is illustrated. In accordance with this variation, the heaterand the temperature sensors are disposed within the conduit and,therefore, in contact with the fluid flowing within the conduit. Inaccordance with this variation the first and second temperature sensorsare disposed at a respective first and second predetermined distancefrom the heater just as in the FIG. 9A embodiment opposite to the fluidflow direction as shown in FIG. 10, or as in the FIG. 9B embodiment inthe fluid flow direction, as shown in FIG. 10.

As in the previous alternative embodiments shown in FIGS. 4A-5, thethermal flow sensor, which has the first and second temperature sensorsdisposed on the same side of the heater, either opposite to the flowdirection or in the flow direction, can also be comprised of twosubstrates 12′ and 14′, as illustrated in FIGS. 4 a, b and c, or withonly one substrate 12″, as shown in FIG. 5.

The present inventors have discovered that the asymmetric sensor designcan not detect flow below a certain flow rate that will be referred toas the cut-off flow rate. The cut-off flow rate is typically about 1 to2 ml/hr. To detect flow from 0 ml/hr up to the cut-off rate, one may usea second heater 56, as illustrated in FIG. 9A. Heater 56 is disposedbetween second sensor 38 and first sensor 36 with respect to the flowdirection.

Having described the presently preferred exemplary embodiment of athermal flow sensor in accordance with the present invention, it isbelieved that other modifications, variations and changes will besuggested to those skilled in the art in view of the teachings set forthherein. Substitutions of elements from one described embodiment toanother are also fully intended and contemplated. It is also to beunderstood that the drawings are not necessarily drawn to scale, butthat they are merely conceptual in nature. It is, therefore, to beunderstood that all such modifications, variations, and changes arebelieved to fall within the scope of the present invention as defined bythe appended claims.

Every issued patent, pending patent application, publication, journalarticle, book or any other reference cited herein is each incorporatedby reference in their entirety.

1. A thermal flow sensor comprising: a first substrate having a firstside and a second opposite side; a second substrate having a first sideand a second opposite side, said first substrate being connected to saidsecond substrate such that said second side of said first substrateabuts said first side of said second substrate; a third substrate havinga first side and a second opposite side, said third substrate beingconnected to said second substrate such that said second side of saidsecond substrate abuts said first side of said third substrate; whereinsaid second substrate having a groove formed therein so as to form aconduit bounded by said second substrate and said second side of saidfirst substrate and said first side of said third substrate, a heaterdisposed on said first side of said first substrate opposed to saidconduit; a first temperature sensor disposed on said first side of saidfirst substrate opposed to said conduit; and a second temperature sensordisposed on said first side of said first substrate opposed to saidconduit; wherein a first recess being formed in at least one of saidfirst side and said second side of said first substrate between saidheater and one of said first temperature sensor and said secondtemperature sensor.
 2. The thermal flow sensor of claim 1, wherein asecond recess being formed in said first side of said first substratebetween said heater and the other one of said first temperature sensorand said second temperature sensor.
 3. The thermal flow sensor of claim1, wherein said first recess extends into said first substrate for abouthalf of the thickness of said first substrate.
 4. The thermal flowsensor of claim 2, wherein a second recess being formed in said firstside of said first substrate between said heater and the other one ofsaid first temperature sensor and said second temperature sensor, saidsecond recess extends into said first substrate for about half of thethickness of said first substrate.
 5. The thermal flow sensor of claim1, wherein said first recess is disposed immediately adjacent to saidheater.
 6. The thermal flow sensor of claim 1, wherein said first recessis disposed immediately adjacent to said heater, a second recess beingformed in said first side of said first substrate between said heaterand the other one of said first temperature sensor and said secondtemperature sensor, said second recess is disposed immediately adjacentto said heater on an opposite side of said heater from said firstrecess.
 7. The thermal flow sensor of claim 1, wherein said first recessis disposed in said first side of said first substrate.
 8. The thermalflow sensor of claim 1, wherein said first recess is disposed in saidsecond side of said first substrate.
 9. The thermal flow sensor of claim1, wherein said first recess is comprised of a first recess and a secondrecess, said first recess is disposed in said first side of said firstsubstrate and said second recess is disposed in said second side of saidfirst recess.
 10. A thermal flow sensor comprising: a first substratehaving a first side and a second opposite side; a second substratehaving a first side and a second opposite side, said first substratebeing connected to said second substrate such that said second side ofsaid first substrate abuts said first side of said second substrate;wherein at least one of said first substrate and said second substratehaving a groove formed therein so as to form a conduit bounded by saidfirst and second substrates; a heater disposed on said first side ofsaid first substrate opposed to said conduit; a first temperature sensordisposed on said first side of said first substrate opposed to saidconduit; and wherein a first recess being formed in at least one of saidfirst side and said second side of said first substrate between saidheater and said first temperature sensor.
 11. The thermal flow sensor ofclaim 10, further comprising a second temperature sensor disposed onsaid first side of said first substrate opposed to said conduit.
 12. Thethermal flow sensor of claim 11, further comprising a second recessformed in said first side of said first substrate between said heaterand said second temperature sensor.
 13. The thermal flow sensor of claim10, wherein said first recess extends into said first substrate forabout half of the thickness of said first substrate.
 14. The thermalflow sensor of claim 12, wherein said second recess entends into saidfirst substrate for about half of the thickness of said first substrate.15. The thermal flow sensor of claim 10, wherein said first recess isdisposed immediately adjacent to said heater.
 16. The thermal flowsensor of claim 12, wherein said first recess is disposed immediatelyadjacent to said heater, said second recess is disposed immediatelyadjacent to said heater on an opposite side of said heater from saidfirst recess.
 17. The thermal flow sensor of claim 10, wherein saidfirst recess is disposed in said first side of said first substrate. 18.The thermal flow sensor of claim 10, wherein said first recess isdisposed in said second side of said first substrate.
 19. The thermalflow sensor of claim 10, wherein said first recess is comprised of afirst recess and a second recess, said first recess is disposed in saidfirst side of said first substrate and said second recess is disposed insaid second side of said first recess.
 20. A thermal flow sensorcomprising: a first substrate having a first upper side and a secondopposite lower side, and at least one side edge extending between saidfirst upper side and said second lower side, a conduit formed thereinhaving an inlet opening and an outlet opening, each of said openingsbeing formed in said at least one edge a heater disposed on said firstside of said first substrate opposed to said conduit; a firsttemperature sensor disposed on said first side of said first substrateopposed to said conduit; and wherein a first recess being formed in atleast one of said first side and said second side of said firstsubstrate between said heater and said first temperature sensor.
 21. Thethermal flow sensor of claim 20, further comprising a second temperaturesensor disposed on said first side of said first substrate opposed tosaid conduit.
 22. The thermal flow sensor of claim 21, furthercomprising a second recess formed in said first side of said firstsubstrate between said heater and said second temperature sensor. 23.The thermal flow sensor of claim 20, wherein said first recess extendsinto said first substrate for about half of the thickness of said firstsubstrate.
 24. The thermal flow sensor of claim 22, wherein said secondrecess extends into said first substrate for about half of the thicknessof said first substrate.
 25. The thermal flow sensor of claim 20,wherein said first recess is disposed immediately adjacent to saidheater.
 26. The thermal flow sensor of claim 22, wherein said firstrecess is disposed immediately adjacent to said heater, said secondrecess is disposed immediately adjacent to said heater on an oppositeside of said heater from said first recess.
 27. A method of determiningthe flow rate of a fluid within a conduit within a body, said bodyhaving a first upper side and a second opposite lower side, and at leastone side edge extending between said first upper side and said secondlower side, a conduit formed within the body having an inlet opening andan outlet opening, each of said openings being formed in said at leastone side edge, said method comprising the steps of: permitting a fluidto enter into the inlet opening of the conduit; heating the fluid with aheater disposed on the first side of the body opposed to and remote fromthe conduit; detecting a temperature of the fluid with a firsttemperature sensor disposed on the first side of the body opposed to andremote, from the conduit; creating a first recess in at least one of thefirst side and second side of the body between the heater and the firsttemperature sensor; and determining the flow rate of the fluid based onthe detected temperature.
 28. The method according to claim 27, whereinthe body comprises a first substrate having a first side and a secondopposite side, and at least a second substrate having a first side and asecond opposite side, said first substrate being connected to saidsecond substrate such that said second side of said first substrateabuts said first side of said second substrate, said second substratehaving a groove formed therein so as to form the conduit bounded by saidsecond substrate and said second side of said first substrate, whereinin said creating step, said first recess is formed in said first side ofsaid first substrate.
 29. The method according to claim 27 furthercomprising the steps of: detecting a temperature of the fluid with asecond temperature sensor disposed on the first side of the body opposedto and remote from the conduit; creating a second recess in at least oneof the first side and the second aide of the body between the heater andthe second temperature sensor.
 30. The method according to claim 29,wherein in said creating step, said first recess and said second recessare formed about half of the thickness of the first substrate.